A Feasibility Study of an Auxiliary Power Unit Based on a PEM Fuel Cell for On-Board Applications

A Feasibility Study of an Auxiliary Power Unit Based on a PEM Fuel Cell for On-Board Applications
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   1 Copyright © 2005 by ASME Proceedings of EFC: 1 st  European Fuel Cell Technology & Application Conference December 14-16, 2005 – Rome, Italy EFC2005-86219 A FEASIBILITY STUDY OF AN AUXILIARY POWER UNIT BASED ON A PEM FUEL CELL FOR ON-BOARD APPLICATIONS M. Bagnoli Università di Bologna – DIEM B. Belvedere Università di Bologna – DIEM M. Bianchi Università di Bologna – DIEM A. Borghetti Università di Bologna – DIE A. De Pascale Università di Bologna – DIEM M. Paolone Università di Bologna – DIE  ABSTRACT PEM (Proton Exchange Membrane) fuel cells show characteristics of high power density, low operating temperature and fast start-up capability, which make them potentially suitable to replace conventional power sources (e.g. internal combustion engines) as Auxiliary Power Units (APU) for on-board applications. This paper presents a methodology for a preliminary investigation on either sizing and operating management of the main components of an on-board power system composed by: i) PEM fuel cell, ii) hydrogen storage subsystem, iii) battery, iv) grid interface for the connection to an external electrical power source when available, and v) electrical appliances and auxiliaries installed on the vehicle. A model able to reproduce the typical profiles of electric power requests of on-board appliances and auxiliaries has been implemented in a computer program. The proposed methodology helps also to define the sizing of the various system components and to identify the fuel cell operating sequence, on the basis of the above mentioned load profiles.   2 Copyright © 2005 by ASME NOMENCLATURE Acronyms   AC Alternate Current APU Auxiliary Power Unit DC Direct Current FC Fuel Cell PEM Proton Exchange Membrane SOC battery State Of Charge Symbols  A i,j  logic variable for on/off state of load  j  at time interval i    B i  battery SOC reduction at time interval i  (kWh)  E  Acc  minimum required battery capacity (kWh)  E  Tsh threshold of battery SOC reduction for FC startup (kWh) i  index of time intervals  j  index of loads  M   number of time intervals  N   number of loads P  j  rated power request of load  j  (kW) P FC i  FC power output at time interval i  (kW) P min  minimum operating value of FC power output (kW) P 0  rated (maximum) value of FC power output (kW)  p i,j  utilization probability of load  j  at time interval i    R i  total load energy request in time interval i  (kWh) T   time window (h) T  net  duration of FC operation (h) T  w  FC startup delay (min)   t   time interval duration (min)   3 Copyright © 2005 by ASME 1. INTRODUCTION The energy dependence on oil of many industrialized countries and the rising barrel price are encouraging studies related to the development of a hydrogen economy. In this framework, the employment of Fuel Cells (FC) is considered as a viable strategy to generate electricity, in view of the possibility of producing hydrogen also from renewable energy sources. Several benefits can be highlighted concerning FC systems. FC are characterized by high energy conversion efficiency values. Moreover, the system performances are not significantly affected by the FC size in a typical range from few kW up to about 200 kW, which allows for modular installation. FC employment leads also to local environmental benefits in comparison with systems based on combustion of fossil fuels. In stationary applications, according to the distributed generation concept, FC systems may replace conventional technologies as emergency back-up units, energy sources integrating the electricity request of grid-connected customers or to supply grid-independent power systems [1]. In on-board applications, such as vehicles (trucks, campers, etc…) and also marine yachts or planes, FC have been proposed to be adopted both to supply electric traction drives and as auxiliary power unit (APU) of the electric loads, when generators moved by the traction engines are not operating and the external network is not available. Low temperature fuel cells are well suited for both on-site stationary or on-board power units [2-6], due to their rather simple and safe operation. Low temperature levels allow fast start-ups and fast responses, suitable for load-following applications. The PEM (Proton Exchange Membrane) fuel cell technology, in particular, seems to be the most competitive solution for on-board applications, because of its very low operating temperature (60-90°C) and compactness. This paper aims at presenting a methodology for a preliminary investigation on either sizing and operating management of the main components of an APU energy system for on-board application based on a PEM fuel cell. Section 2 describes the considered energy system. Section 3 presents the proposed methodology, implemented in a computer program, able to reproduce typical electric power request profiles and to define the most adequate sizing of the APU components and to identify the FC operating sequence. Section 4 shows some of the results obtained for a typical configuration of a medium size motor yacht. 2. APU BASED ON A FC FOR ON-BOARD APPLICATIONS As already mentioned, the energy system considered in this study is an APU based on a FC, devoted to the supply of several small electric appliances and auxiliaries installed in a vehicle. As shown in Fig. 1, the system is composed by: i) PEM fuel cell, ii) hydrogen storage subsystem, iii) grid interface for the connection to an external power source, when available, iv) battery and v) on-board (AC and DC) loads. The external power source may be represented either by a generator moved by the traction engines, when operating, or by the external network, when available.   4 Copyright © 2005 by ASME FC stack   External energy source Power conditioning system H2 storage Cooling   Syst em   Air Inlet system Loads and battery Fig. 1 – Main components and energy fluxes of the considered energy system A simplified scheme of the electric system is shown in Fig. 2. The FC and the battery are connected to the main AC bus through power electronic interfaces. This solution would reduce the mutual interference between the two energy sources. The same AC bus is also fed by the external source when available. When the connection with such an external source is not available, the power electronic interface of the battery assumes the main role on the control of the electric power characteristics of the system. (80-50 V)   FC AC loads (230 V)  DC loads (24 V)  external source inverter and charger battery DC bus   single phase AC bus   DC   DC   (42 V) DC/DC booster DC/AC inverter Fig. 2 – Scheme of the electric system 3. METHODOLOGY A computational procedure has been developed in order to define the size of the APU components and to decide the operating sequence of the FC, once the main characteristics of the appliances and auxiliaries to be supplied are defined. The developed   5 Copyright © 2005 by ASME procedure (illustrated in Fig. 3) has been implemented in a numerical code, which simulates the random occurrence of the utilities request. The procedure takes into account the possibility for the system to be fed by external source, as it usually happens when the system is connected to the external electric grid, or when traction engines are in operation and move an electric generator. The proposed procedure is based on the assumption that the FC operation (i.e. full-load, part-load or disconnected) is decided on the basis of the battery State Of Charge (SOC); therefore, the FC operation is not based on a load-following strategy. Thus, three different operating conditions of the APU are identified: •   connection with external source available: in this case the loads are expected to be fully fed by the external source, being the FC switched off and the battery in charge; •   external source not available and FC not operating: in this case the energy required by both DC and AC loads is provided by the battery; •   external source not available and FC switched on: in this case the loads are completely or partially fed by the energy provided by the FC, while the unbalances between load request and FC production are compensated by the battery. In case of high load peaks, which cannot be met by the available resources, the intervention of a specific protection device is required. load data operating scenario typical load profiles FC data FC on-off operating sequence   number of FC start-ups minimum battery size   hydrogen requirements P 0 P min   T  w    N P  j   p i,j  T T  net    M t  i    R i  P FC i    B i   random number generation   Fig. 3 - Flow chart of the proposed procedure
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